Tyrosine phosphorylation of intracellular proteins is stimulated by several external factors that influence cell growth. One of these is cell-extracellular matrix adhesion which initiates signals through integrin receptors to affect cytoskeletal assembly, cell cycle progression, and gene expression. Sites of cell-substrate adhesion, focal adhesions, are enriched in phosphotyrosine. Assembly of focal adhesions and anchorage-dependent proliferation require tyrosine phosphorylation. However, elevated levels of phosphotyrosine in focal adhesions correlate with a transformed phenotype such as that of v-src transformed cells. These cells display anchorage-independent growth, a rounded cell morphology, and poorly organized focal adhesions and cytoskeleton. In addition, the turnover of focal adhesions in mitosis coincides with changes in tyrosine phosphorylation of focal adhesion proteins. The balance of tyrosine phosphorylation in focal adhesions is likely controlled by protein tyrosine phosphatases (PTPs). The major objectives of the proposed work are to identify PTPs associated with focal adhesions and to determine their functional role in focal adhesion assembly, anchorage-dependent proliferation, and turnover of focal adhesions during mitosis. Using an in-gel PTP assay, PTPs co- immunoprecipitating with focal adhesion proteins will be identified, cloned, and functionally characterized.